Mechanical delousing and other handling procedures are known to trigger acute stress in Atlantic salmon (Salmo salar), leading to reduced appetite, growth losses, and a weakened physiological condition. In response to this challenge,
a new functional diet has been developed to improve recovery following stressful production events. The feed includes added magnesium and selected plant raw materials intended to support post-stress appetite and growth.
A 9-month trial was conducted at BioMar's research facility LetSea (Norway) to evaluate the efficacy of this functional feed under realistic production conditions. A challenge model simulating the delousing process was developed, involving mild crowding (1.5 hours), sedation, netting, individual weighing, and manual delousing. This procedure
was repeated four times during the trial. The experimental diet was fed for two weeks prior to each handling event and continued during the week of treatment. Atlantic salmon were followed from 1.5 kg to 4.3 kg final weight. A control feed was used in both stressed and unstressed groups for performance comparison.
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The handling procedure reliably induced a physiological stress response. Cortisol levels in fish scales were significantly higher in stressed groups, along with shifts in blood serum biomarkers such as lactate, glucose, and key electrolytes (magnesium, calcium, potassium). Liver samples showed upregulation of stress-related genes, and morphological changes were recorded in heart condition and cardio somatic index (CSI). Stressed fish displayed higher CSI values and narrower hearts, and external signs of physical stress such as skin hemorrhages and snout wounds were more pronounced.
Fish fed the functional diet exhibited improved feed intake following handling events, as indicated by higher thermal growth coefficients (TGC) compared to the stressed control group. Despite no significant differences in cortisol or serum stress markers between stressed groups, fish on the functional feed demonstrated a more favourable post-stress growth trajectory. Appetite stimulation appears to be the key driver behind this recovery effect.
Despite no significant differences in cortisol or serum stress markers between stressed groups, fish on the functional feed demonstrated a more favourable post-stress growth trajectory. Appetite stimulation appears to be the key driver behind this recovery effect.
Fillet colour was enhanced in the functional feed group, reflecting improved feed consumption during recovery periods. Increased pigmentation is consistent with elevated intake of carotenoid-rich diets, and therefore indicates that fish returned to normal feeding behaviour more rapidly post-handling.
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This study confirms that the tested challenge model is effective in simulating real-world stress events such as mechanical delousing. The functional diet tested did not suppress the normal stress response, but rather supported a faster return to normal
appetite and growth. By reducing post-handling growth losses and improving fillet quality, the feed offers a practical tool for improving robustness in commercial salmon production.
Results (1)
This study explored how different levels of synthetic vitamin D3 (cholecalciferol) supplementation influence whole body and fillet vitamin D3 deposition in rainbow trout reared under recirculating aquaculture system (RAS) conditions.